1. Field of the Present Invention
The present invention relates generally to a power converter and power conversion method, and more particularly, to a system and method for controlling a duty cycle of a switching power supply.
2. Description of the Related Art
Due to the different power requirements of various applications, power converters, such as switching power supplies, are widely employed in the electrical technical field to supply desired voltages/currents. The switching power supply maintains a stable voltage or current output at an output side of a power conversion circuit by controlling the on/off (and thus a duty cycle) of a switching circuit at an input side of the power conversion circuit. Such power conversion circuit may be implemented, for example, with a transformer, which converts an input voltage received at a primary side into an output voltage and current at a secondary side.
In one aspect, it is desirable to improve the efficiency of such power converter, e.g., the ratio of the output power to the input power. In another aspect, it is desirable for such power converter to have a high power density, e.g., the ratio of the output power to the volume of the power converter. In yet another aspect, it is desirable for a power converter to have good Line Regulation and/or Load Regulation. Line Regulation, also known as Input Voltage Regulation, refers to the percentage of the output voltage deviating from the nominal output voltage when the input voltage varies in a full input range. Load Regulation refers to the variation of the output voltage when the load increases from zero to a nominal value under a fixed input voltage.
In the prior art, some power converters have fixed output voltages, and thus may have relatively good Line Regulation and Load Regulation. However, since the duty cycle of the switching circuit may have to be adjusted within a wide range depending on the variation of the input voltage in order to maintain the fixed output voltages, the circuit design is complicated and thus may be unable to satisfy the requirements of high efficiency and high power density. Yet some power converters use a fixed duty cycle for the switching circuit, e.g., a duty cycle of approximately 50%. Such a switching power supply may employ a self-driven synchronization rectifier on the secondary side, and thus the output inductance may be designed to be very small, thereby achieving a relatively high efficiency and high power density. However, with a fixed duty cycle, the output voltage changes along with the input voltage, likely resulting in poor Line Regulation and/or Load Regulation.